The present invention relates to a loudspeaker unit, particularly for reproduction of sound of low frequences, although such a unit may of course also be equipped with one or more drivers for higher frequences.
It is a traditional problem that a base loudspeaker, for correctly reproducing the low sound frequences, should be mounted on an extremely large baffle plate in order to prevent the air pressure variations at the front side of the loudspeaker diaphragm from being more or less suppressed by the opposite or counter phase air pressure variations as occurring at the rear side of the diaphragm. The practical solution of this problem is the use of a box structure having a front wall, on which the loudspeaker is mounted, and the resulting loudspeaker unit may be either of the closed type, in which the box or cabinet is entirely closed, or of the so-called bass reflection type, in which the box has an opening spaced from the loudspeaker, whereby the "rear slide sound" of the loudspeaker is radiated through said opening or acoustic port without giving rise to considerable counter phase problems. These two basic solutions each show certain advantages and certain disadvantages as to the quality and the volume of the reproduced sound. Thus, the closed systems are known to produce a high sound quality, while the bass reflection systems are known to be of a high efficiency.
Another traditional problem is that for achieving a high sound volume or intensity it is necessary to use a large and rather expensive loudspeaker, and it is well known that a cheaper and otherwise more convenient solution may be to use two smaller loudspeakers. These may both be mounted on the front plate of the cabinet whereby they operate acoustically in parallel, without requiring any very large width of the front plate, or they may be mounted one behind the other in a so-called compound system, in which they operate acoustically in series. An associated problem, however, is that the combined moved masses of the two drivers is relatively high, whereby the entire unit is subjected to pronounced mechanical vibrations, which may very well lead to a displacement and a fall down of a loudspeaker unit placed e.g. on a shelf.
The present invention has for its purposes to provide a loudspeaker unit in which two loudspeakers for low frequency reproduction are arranged in a novel and advantageous manner.
According to the invention the two loudspeakers are arranged, as already known, one behind the other, but connected so as to operate in counter phase, one loudspeaker being mounted on the front plate of the cabinet and the other loudspeaker being mounted on an internal partition plate inside the cabinet, said partition plate dividing the cabinet in a front chamber, which is provided with an acoustic port, and a rear chamber, which is basically a closed chamber. With this arrangement the diaphragms of the two loudspeakers will cooperate to produce the air pressure variations in the said front chamber and thus condition the latter to function as a very effective bass reflection system, with the front loudspeaker still radiating the sound in a direct manner from the front side of its diaphragm. The diaphragm of the rear loudspeaker will work rearwardly into the rear closed chamber, whereby the diaphragm is influenced by the associated operation conditions of a closed system, as normally providing for a high quality of the sound reproduction due to an effective reproduction of even very low frequences. This implies that the contribution of the rear loudspeaker to the sound production in the front chamber will be adjusted to an improved sound quality as compared with the front loudspeaker, and the result will be a loudspeaker unit or system, which is a highly effective bass reflection system capable of reproducing the sound with a remarkably improved sound quality.
Another remarkable advantage of the system according to the invention is that the two loudspeakers as placed one behind the other and operating in counter phase, geometrically, will dynamically outbalance each other, such that the resulting mechanical vibrations of the entire unit will be very small if not totally eliminated.
In a preferred embodiment the two loudspeakers are oriented back to back, i.e. with their rear magnetic systems facing each other, and a rigid mechanical connection is established directly between the magnetic systems, whereby practically no vibrations are transferred to the respective mounting plates of the cabinet. It will be readily understood by those skilled in the art that it is hereby even possible to make use of loudspeakers having a mechanically rather weak chassis located between the magnetic system and the outer chassis rim portion as supporting the other edge of the diaphragm and being used for securing the loudspeaker to its mounting plate, i.e. the loudspeakers may even be of a relatively cheap construction.
When the two loudspeakers are stabilized by means of the said rigid interconnection the outer rim portions of the loudspeaker chassises should not necessarily be rigidly fastened to the respective mounting plates, and according to the invention, therefore, the loudspeaker sub unit as constituted by the two interconnected loudspeakers may advantageously be mounted in the cabinet by the intermediate of resilient holding means interposed between the outer chassis rim portions of the loudspeakers and the edges of the respective holes in the mounting plates. Hereby any possible resulting vibrations of the said loudspeaker sub unit will be absorbed by the resilient holding means, such that the entire unit will be practically completely non-vibrating.
The present invention further relates to a loudspeaker system of the so-called compound type, i.e. in which two principally similar loudspeakers cooperate to reproduce the same frequency range, such that relatively large volumes of air are to be oscillated. Bass loudspeakers are usually rather large units for fulfilling this purpose, but for various reasons that are practical and economical limitations as to their size, just as very large loudspeakers will require correspondingly broad loudspeaker cabinets. These problems may be overcome with the use of doubled loudspeakers, which, already by their mounting side by side or above each other may account for a more suitable shape of the loudspeaker cabinet than with the use of a single loudspeaker of the same, combined diaphragm area, just as such a single loudspeaker will normally, when made with a high quality, be more than twice as expensive as each of the two singular loudspeakers.
The two cooperating loudspeakers may be arranged in two principally different manners for achieving two different sets of advantages. With the said arrangement of the loudspeakers side by side or in parallel a large total diaphragm area will be achieved and therewith a high efficiency, but the cabinet shall have to be correspondingly large, and thus the main effect is an increase of the efficiency of the loudspeaker unit. It is well known that with the use of bass loudspeakers some--normally accepted--distortion phenomina occur near the relevant resonance frequency as a consequence of the effect onto the rear side of the diaphragm of the air vibrations inside the cabinet, and this effect will not be changed with the use of two loudspeakers mounted side by side in the cabinet.
In a "genuine" compound system use is made of another possibility, viz. by arranging the two loudspeakers acoustically in series, i.e. one behind the other, with the space between the loudspeakers closed by means of a cylinder extending between the opposed peripheries of the respective two loudspeaker diaphragms. The diaphragms will move forwardly and rearwardly in phase with each other, whereby the foremost diaphragm will not meet with any air spring effect from the inside, i.e. the foremost, front radiating loudspeaker may work with a high efficiency and thus produce a more powerfull bass than a corresponding singular loudspeaker, though less powerfull than with the use of two loudspeakers arranged in parallel. The cabinet is connected with but a single rear side of the diaphragm, i.e. the cabinet may be as small as adapted to only a single loudspeaker.
Besides, the said distortions as due to influence onto the diaphragm side in contact with the air of the cabinet, will occur only at the rear diaphragm, because they are only to a very small extent transferred to the foremost, front radiating diaphragm through the restricted air space between the diaphragms. Thus, both a relatively non-distorted and yet rather strong bass reproduction may be obtained, even with the use of a relatively small cabinet.
However, both of the said systems, which are used all according to the desired sets of advantages, show a common substantial drawback, viz. that the cabinet is subjected to quite considerable oscillations from the oscillating systems of the loudspeakers, this giving rise to high requirements with respect to the construction and the anchoring of the cabinet.
It is a purpose of the invention to provide a loudspeaker system of the component type, in which the oscillations of the cabinet can be eliminated regardless of the loudspeakers being mounted in series or in parallel, and in which a good efficiency is achievable.
According to the invention the two loudspeakers are mounted back to back, i.e. with their associated magnet systems facing each other and coupled together in a mechanically rigid manner, while their voice coils are connected to oscillate in mutual counterphase, such that in operation type will move towards and away from each other. An immediate result is that the loudspeaker or driver unit as such will not transfer noticeable oscillations to the cabinet, inasfar as noticeable resulting oscillations will simply not be created by the counterphase motion of the movable parts of the respective loudspeakers, when these are similar in the relevant manner. Another achievable advantage is that the rear sides of the magnet systems, when mounted close to each other, may affect each other magnetically such that with a uniform polarity in the two systems the said rear sides will be mutually repellant due to the stray flux adjacent and outside the rear sides. It is already a known fact that it is advantageous to mount an inversely polarized magnet at the rear side of the magnetic system of a loudspeaker, as the resulting repulsion will intensify the magnetic field at the front or operative end of the magnetic system, i.e. in the annular gap, in which the voice coil operates. It will be appreciated that in accordance with the invention there is not just added a magnet, but the magnetic system of an extra loudspeaker, whereby the two loudspeakers affect each other to increase both of the operative magnetic fields adjacent the voice coils, without the use of separate, extra magnets for that purpose. With a correct polarization of the two magnetic systems it will thus be obtained as an additional advantage that the efficiency of both loudspeakers will be increased.
The said building together of the loudspeakers back to back may be applied regardless of whether they should operate, acoustically, in series or in parallel. In the two instances, however, different acoustic connections should be provided, and the two situations should here be commented upon separately.
1. Series Connection. Here advantage is taken of the fact that the space behind the front radiating diaphragm will expand and contract in synchronism with the corresponding respective contraction and expansion of the space behind the rear diaphragm by the counterphase movements of the voice coils, i.e. with a suitable shielding and mutual connection of these spaces a closed chamber may be provided, in which the air can pulsate in an alternating manner without showing any substantial air spring action on the diaphragms. The sound will still be radiated from the front side of the front diaphragm, while the effective acoustic connection between the combined loudspeaker unit and the interior of the cabinet exist at the front facing side of the rear diaphragm and not as in the prior art at the rear side of the diaphragm, as this side is now cooperating with the said closed chamber.
The flow connection between the space behind the front diaphragm and the space behind the rear diaphragm may well be established through external channel means, but it is a special feature of the invention that it is possible to use a central channel, inasfar as the central portions of the adjacent magnetic systems may be shaped in a tubelike manner, such that the air may flow directly through the central magnetic system itself.
2. Parallel Connection. Here the movements of the rear diaphragm should be transmitted forwardly to support the radiation from the front side of the front diaphragm, i.e. in phase therewith. Due to the counterphase coupling it will be the space behind the rear diaphragm which will acoustically correspond to the space in front of the front diaphragm, and the parallel connection, therefore, can be established by isolating or shielding the space behind the rear diaphragm and connecting it through a sound channel to the front side of the cabinet, while the cabinet chamber is in open connection with the rear side of the front diaphragm and the front side of the rear diaphragm, respectively. In a preferred embodiment use is even here made of the said tubelike design of the magnetic systems of the loudspeakers, this tube being at disposal as the said sound channel, when also the front diaphragm is open or apertured in its associated middle area. The sound from the rear loudspeaker will thus be front radiated through a middle hole in the diaphragm of the front loudspeaker, whereby a certain attenuation of the sound will occur when compared with the front radiation of an extra, front mounted loudspeaker, but the sound nevertheless originates from the space behind the entire rear side of the rear diaphragm, i.e. the sound energy of the air will be considerably higher than the energy radiated by a diaphragm just having the size of the said central aperture of the front diaphragm.
In the following the invention is described in more detail with reference to the drawing, in which: